Photoacoustic tomography (PAT) can create multiscale multicontrast images of living biological structures ranging from organelles to organs. This emerging technology overcomes the high degree of scattering of optical photons in biological tissue by making use of the photoacoustic effect. Light absorption by molecules creates a thermally induced pressure jump that launches ultrasonic waves, which are received by acoustic detectors to form images. Different implementations of PAT allow the spatial resolution to be scaled with the desired imaging depth in tissue while a high depth-to-resolution ratio is maintained. As a rule of thumb, the achievable spatial resolution is on the order of 1/200 of the desired imaging depth, which can reach up to 7 centimeters. PAT provides anatomical, functional, metabolic, molecular, and genetic contrasts of vasculature, hemodynamics, oxygen metabolism, biomarkers, and gene expression. We review the state of the art of PAT for both biological and clinical studies and discuss future prospects.

Photoacoustic Tomography: In Vivo Imaging from Organelles to Organs

We describe a fully-vectorial, three-dimensional algorithm to compute the definite-frequency eigenstates of Maxwell's equations in arbitrary periodic dielectric structures, including systems with anisotropy (birefringence) or magnetic materials, using preconditioned block-iterative eigensolvers in a planewave basis. Favorable scaling with the system size and the number of computed bands is exhibited. We propose a new effective dielectric tensor for anisotropic structures, and demonstrate that O Delta x;2 convergence can be achieved even in systems with sharp material discontinuities. We show how it is possible to solve for interior eigenvalues, such as localized defect modes, without computing the many underlying eigenstates. Preconditioned conjugate-gradient Rayleigh-quotient minimization is compared with the Davidson method for eigensolution, and a number of iteration variants and preconditioners are characterized. Our implementation is freely available on the Web.

Block-iterative frequency-domain methods for Maxwell’s equations in a planewave basis

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Photoacoustic imaging (also called optoacoustic or thermoacoustic imaging) has the potential to image animal or human organs, such as the breast and the brain, with simultaneous high contrast and high spatial resolution. This article provides an overview of the rapidly expanding field of photoacoustic imaging for biomedical applications. Imaging techniques, including depth profiling in layered media, scanning tomography with focused ultrasonic transducers, image forming with an acoustic lens, and computed tomography with unfocused transducers, are introduced. Special emphasis is placed on computed tomography, including reconstruction algorithms, spatial resolution, and related recent experiments. Promising biomedical applications are discussed throughout the text, including (1) tomographic imaging of the skin and other superficial organs by laser-induced photoacoustic microscopy, which offers the critical advantages, over current high-resolution optical imaging modalities, of deeper imaging depth and higher absorptioncontrasts, (2) breast cancerdetection by near-infrared light or radio-frequency–wave-induced photoacoustic imaging, which has important potential for early detection, and (3) small animal imaging by laser-induced photoacoustic imaging, which measures unique optical absorptioncontrasts related to important biochemical information and provides better resolution in deep tissues than optical imaging.

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Photoacoustic imaging in biomedicine

Photoacoustic (PA) imaging, also called optoacoustic imaging, is a new biomedical imaging modality based on the use of laser-generated ultrasound that has emerged over the last decade. It is a hybrid modality, combining the high-contrast and spectroscopic-based specificity of optical imaging with the high spatial resolution of ultrasound imaging. In essence, a PA image can be regarded as an ultrasound image in which the contrast depends not on the mechanical and elastic properties of the tissue, but its optical properties, specifically optical absorption. As a consequence, it offers greater specificity than conventional ultrasound imaging with the ability to detect haemoglobin, lipids, water and other light-absorbing chomophores, but with greater penetration depth than purely optical imaging modalities that rely on ballistic photons. As well as visualizing anatomical structures such as the microvasculature, it can also provide functional information in the form of blood oxygenation, blood flow and temperature. All of this can be achieved over a wide range of length scales from micrometres to centimetres with scalable spatial resolution. These attributes lend PA imaging to a wide variety of applications in clinical medicine, preclinical research and basic biology for studying cancer, cardiovascular disease, abnormalities of the microcirculation and other conditions. With the emergence of a variety of truly compelling in vivo images obtained by a number of groups around the world in the last 2–3 years, the technique has come of age and the promise of PA imaging is now beginning to be realized. Recent highlights include the demonstration of whole-body small-animal imaging, the first demonstrations of molecular imaging, the introduction of new microscopy modes and the first steps towards clinical breast imaging being taken as well as a myriad of in vivo preclinical imaging studies. In this article, the underlying physical principles of the technique, its practical implementation, and a range of clinical and preclinical applications are reviewed.

/pdf/biomedical-photoacoustic-imaging-4s982wv96j.pdf

Biomedical photoacoustic imaging

A "quantum graph" is a graph considered as a one-dimensional complex and equipped with a differential operator ("Hamiltonian"). Quantum graphs arise naturally as simplified models in mathematics, physics, chemistry, and engineering when one considers propagation of waves of various nature through a quasi-one-dimensional (e.g., "meso-" or "nano-scale") system that looks like a thin neighborhood of a graph. Works that currently would be classified as discussing quantum graphs have been appearing since at least the 1930s, and since then, quantum graphs techniques have been applied successfully in various areas of mathematical physics, mathematics in general and its applications. One can mention, for instance, dynamical systems theory, control theory, quantum chaos, Anderson localization, microelectronics, photonic crystals, physical chemistry, nano-sciences, superconductivity theory, etc. Quantum graphs present many non-trivial mathematical challenges, which makes them dear to a mathematician's heart. Work on quantum graphs has brought together tools and intuition coming from graph theory, combinatorics, mathematical physics, PDEs, and spectral theory. This book provides a comprehensive introduction to the topic, collecting the main notions and techniques. It also contains a survey of the current state of the quantum graph research and applications.

Introduction to Quantum Graphs

A quantum graph is a graph equipped with a self-adjoint differential or pseudo-differential Hamiltonian. Such graphs have been studied recently in relation to some problems of mathematics, physics and chemistry. The paper has a survey nature and is devoted to the description of some basic notions concerning quantum graphs, including the boundary conditions, self-adjointness, quadratic forms, and relations between quantum and combinatorial graph models.

/pdf/quantum-graphs-i-some-basic-structures-2sf0fhd9ca.pdf

Quantum graphs: I. Some basic structures

1. Holomorphic Fredholm Operator Functions.- 1.1. Lifting and open mapping theorems.- 1.2. Some classes of linear operators.- 1.3. Banach vector bundles.- 1.4. Fredholm operators that depend continuously on a parameter.- 1.5. Some information from complex analysis.- A. Interpolation of entire functions of finite order.- B. Some information from the complex analysis in several variables.- C. Some problems of infinite-dimensional complex analysis.- 1.6. Fredholm operators that depend holomorphically on a parameter.- 1.7. Image and cokernel of a Fredholm morphism in spaces of holomorphic sections.- 1.8. Image and cokernel of a Fredholm morphism in spaces of holomorphic sections with bounds.- 1.9. Comments and references.- 2. Spaces, Operators and Transforms.- 2.1. Basic spaces and operators.- 2.2. Fourier transform on the group of periods.- 2.3. Comments and references.- 3. Floquet Theory for Hypoelliptic Equations and Systems in the Whole Space.- 3.1. Floquet - Bloch solutions. Quasimomentums and Floquet exponents.- 3.2. Floquet expansion of solutions of exponential growth.- 3.3. Completeness of Floquet solutions in a class of solutions of faster growth.- 3.4. Other classes of equations.- A. Elliptic systems.- B. Hypoelliptic equations and systems.- C. Pseudodifferential equations.- D. Smoothness of coefficients.- 3.5. Comments and references.- 4. Properties of Solutions of Periodic Equations.- 4.1. Distribution of quasimomentums and decreasing solutions.- 4.2. Solvability of non-homogeneous equations.- 4.3. Bloch property.- 4.4. Quasimomentum dispersion relation. Bloch variety.- 4.5. Some problems of spectral theory.- 4.6. Positive solutions.- 4.7. Comments and references.- 5. Evolution Equations.- 5.1. Abstract hypoelliptic evolution equations on the whole axis.- 5.2. Some degenerate cases.- 5.3. Cauchy problem for abstract parabolic equations.- 5.4. Elliptic and parabolic boundary value problems in a cylinder.- A. Elliptic problems.- B. Parabolic problems.- 5.5. Comments and references.- 6. Other Classes of Problems.- 6.1. Equations with deviating arguments.- 6.2. Equations with coefficients that do not depend on some arguments.- 6.3. Invariant differential equations on Riemannian symmetric spaces of non-compact type.- 6.4. Comments and references.- Index of symbols.

https://iopscience.iop.org/article/10.1070/RM1982v037n04ABEH003965/pdf

Floquet Theory for Partial Differential Equations

The limited-view problem is studied for thermoacoustic tomography, which is also referred to as photoacoustic or optoacoustic tomography depending on the type of radiation for the induction of acoustic waves. We define a “detection region,” within which all points have sufficient detection views. It is explained analytically and shown numerically that the boundaries of any objects inside this region can be recovered stably. Otherwise some sharp details become blurred. One can identify in advance the parts of the boundaries that will be affected if the detection view is insufficient. If the detector scans along a circle in a two-dimensional case, acquiring a sufficient view might require covering more than a π-, or less than a π-arc of the trajectory depending on the position of the object. Similar results hold in a three-dimensional case. In order to support our theoretical conclusions, three types of reconstruction methods are utilized: a filtered backprojection (FBP) approximate inversion, which is shown to work well for limited-view data, a local-tomography-type reconstruction that emphasizes sharp details (e.g., the boundaries of inclusions), and an iterative algebraic truncated conjugate gradient algorithm used in conjunction with FBP. Computations are conducted for both numerically simulated and experimental data. The reconstructions confirm our theoretical predictions.

/pdf/reconstructions-in-limited-view-thermoacoustic-tomography-1wx17z6vao.pdf

Reconstructions in limited-view thermoacoustic tomography

The paper presents a survey of mathematical problems, techniques, and challenges arising in the Thermoacoustic (also called Photoacoustic or Optoacoustic) Tomography.

/pdf/mathematics-of-thermoacoustic-tomography-4454wdhd1p.pdf

Peter Kuchment

Papers

Introduction to Quantum Graphs

Quantum graphs: I. Some basic structures

Floquet Theory for Partial Differential Equations

Reconstructions in limited-view thermoacoustic tomography

Mathematics of thermoacoustic tomography